Head mounted image displays (e.g. helmets, goggles, and eyeglasses incorporating miniature displays) and other compact display systems which provide data in alphanumeric, video, or graphic form have applications in avionics, medicine, entertainment, and wearable computers, as well as numerous other fields. See for example U.S. Pat. Nos. 5,348,477, 5,281,960, 4,806,001 and 5,162,828. There are three principal types of prior art head mounted display systems: "see-through systems," in which the displayed electronic image is combined with the ambient imagery so that the user can see both images; "see-around systems" in which the displayed image occludes a part of the ambient imagery; and "full-immersion systems" in which the entire ambient image is blocked, so that the user sees only the electronically generated image. All three types of systems use various means, including lenses and the like, to project the image into the viewer's eyes.
The simplest systems are of the see-around type in which the electronic display is provided with one or more lenses and suspended in front of the user's eyes. A principal limitation of the device is that the display and optical system must be moved with respect to the head, or the head must be moved, to enable the user to see ambient imagery in the occluded field. A second limitation of such devices is that the device is suspended from the head (or helmet, strap or other support borne by the head), so that the mass of apparatus adds an undesirable weight and/or torque to the head. A third limitation of the device is that position of the exit pupil of the optical system cannot be fixed accurately, meaning that the exit pupil of the optical system must be large enough to accommodate various motions of the device that occur during use.
Full-immersion systems have many of the same limitations as see-around systems. The head mounted system must be removed to view any ambient imagery. Typically, the systems comprise displays and lens systems similar to the see-around display, or comprise a display, lens system and reflective screen. These systems involve high weight, torque and volume.
See-through systems involve the most complex optical designs. Generally, the see-through system comprises a display, lens system, and viewing screen or combiner. All of the limitations of the see-around display are shared by the see-through display, except for the need to remove the head-mounted system to see ambient images. However, for this benefit, it is necessary to add further optical components and thus weight to the system.
All three of the above head mounted display types have the further limitation of requiring that the optical systems be mounted in goggles, helmets, strap-on bands, unusually bulky sunglasses frames having large visors and the like, rather than more conventional optical supports (such as the more simple support provided by conventional eyeglass frames). This limitation requires users to become accustomed to wearing such devices.
Another limitation of prior art displays is the need to provide illumination. For example, head mounted display systems using liquid crystal displays require lamps to illuminate the display. These lamps consume power and generate heat near the user's head and add to the overall volume and weight of the system.
A further and key limitation of the prior art is the use of optical paths external to the eyeglasses system. For example, in U.S. Pat. No. 5,348,477, Welch describes a system comprising an image relay and a set of lenses and screens mounted external to an eyeglass frame and eyeglass lens. The use of a free space optical path, combiners and the like make miniaturization in a form approaching conventional eyeglasses quite difficult. Furness et al., in U.S. Pat. No. 5,162,828, have attempted to address this limitation with a see-through system based on a transparent screen, such as found in a goggle, with a display located at the top of the goggle or eyeglasses, and a mirror, which may be fixed or adjustable, located at the bottom of the transparency. This approach shows reduced complexity, but the system still requires at least one mirror positioned in an unconventional exposed location below or behind the transparency. The eyeglasses disclosed by Perera (U.S. Pat. Nos. 4,867,551 and 4,751,691) and Bettinger (U.S. Pat. No. 4,806,011) also require mirrors suspended from eyeglass frames A particular limitation of systems with mechanically suspended mirrors results from the fact that such appendages have a greater probability that the optical surface of the mirror will suffer damage during use, or will break off, or will damage the user's eye upon accidental impact. Additionally, because these systems include unusual optical appendages, they do not approach the ideal form of conventional eye wear.